1. Field of the Invention
The present invention relates to a color picture tube for use in a television set, a computer display, etc., and particularly to a fixing part for fixing a frame that holds a shadow mask to a glass panel.
2. Related Art
FIG. 1 is a front view of a conventional general color picture tube, cut on a plane that is perpendicular to its tube axis, so as to remove a front part of a glass panel.
As shown in FIG. 1, a frame 106 that holds a shadow mask 104 is elastically fixed, via three support springs 108, 110, and 112, to the inside of a skirt part 102 that is substantially in a rectangular frame-shape and that extends from a front part (not shown) of a glass panel 100. Hereafter in this specification, an axis that is perpendicular to the tube axis and is substantially parallel to a long side of the skirt part is assumed to be a horizontal axis (X axis), and an axis that is perpendicular to both the tube axis and the horizontal axis is assumed to be a vertical axis (Y axis).
The frame 106 is in a rectangular frame-shape and is smaller than the skirt part 102. The support springs 108 and 110 are bonded at its one ends by welding, respectively to two short sides of the frame 106, and the support spring 112 is bonded by welding at its one end, to one long side of the frame 106.
Each of the support springs 108, 110, and 112 is a long and narrow plate, and has an aperture (not shown) at its one end opposite to the end bonded by welding as described above. On the other hand, panel pins 114, 116, and 118 are provided at the inside surface of the skirt part 102 of the glass panel 100, so as to respectively correspond to the support springs 108, 110, and 112. By engaging the apertures of the corresponding support springs 108, 110, and 112 with the panel pins 114, 116, and 118, the frame 106 that holds the shadow mask 104 can be fixed to the inside of the glass panel 100.
There are several methods for arranging such panel pins that are used to fix a frame to the inside of a glass panel. The above-described method of using three pins is the simplest among all, and is typically employed for compact tubes of 21 inches (51 cm) or smaller. One reason for that is as follows. A shadow mask and a frame of such a compact color picture tube are small in size and weight. Therefore, even when the frame holding the shadow mask is fixed to a glass panel simply via three pins, the frame is not likely to be moved to a wrong position with respect to the glass panel in a drop test of the color picture tube.
In a rectangular area of the shadow mask 104 indicated by reference numeral 120, a large number of regularly arranged holes are formed. On the other hand, a phosphor screen formed by regularly arranging red, green, and blue phosphor dots is provided at the inner surface of the front part of the glass panel 100. At the back in the paper, an electron gun (not shown) is provided, and three electron beams emitted from the electron gun are sorted by the holes formed in the shadow mask 104, so that each electron beam hits a phosphor dot of its targeted color.
Here, a ratio of the electron beams passing through the holes is usually as small as 15 to 25%. A large portion of the electron beams collides with solid parts (non-hole parts) of the shadow mask. As a result, heat is generated in the shadow mask, and the generated heat is conducted to the frame, the support springs, and the panel pins in the stated order, thereby causing thermal expansion of each of these components.
Such thermal expansion further causes the holes of the shadow mask 104 to be moved from the correct positions. Along with this, the landing positions on the phosphor screen at which the electrons beams hit are deviated from the correct landing positions. In this specification, the phenomenon that the actual landing positions of the electron beams on the phosphor screen are deviated from the correct landing positions is referred to as “mislanding”. Also, an amount by which the actual landing positions are deviated from the correct landing positions is referred to as a “mislanding amount”.
Along with the increased electron beam current due to higher brightness of color picture tubes, the above-described phenomenon of “mislanding” has become a serious problem in recent years.
For example, an amount of electron beam current is increased due to the following settings often employed in recent years. For computer display monitors, the operating conditions may often be set such that the display is in a reverse-mode where a background of a display screen is white, brightness of the center of the display screen is as high as 100 cd/cm2, and the display size is “full scan”.
As a result, it has been extremely difficult to prevent mislanding caused by thermal expansion occurring in a greater scale than thermal expansion occurring in conventional cases, even if an INVAR material (“INVAR” is a trademark) having a low thermal expansion coefficient is employed for a shadow mask. Such a phenomenon occurs that the shadow mask is moved unsymmetrically in the left-right direction with respect to the Y axis at the time of entire doming. The mislanding causes such problems as degradation in brightness of the display screen due to the degraded luminous efficiency of the phosphors, and deterioration in the while color uniformity on the display screen.
In view of these problems, a technique is proposed for reducing the mislanding by employing the structure where a welding spot at which the shadow mask and the frame are welded together is set away from the central axis of the shadow mask (see Japanese Laid-open Patent Application No. H10-321153). With this technique, however, the effect of reducing the mislanding can be obtained only in the vicinity of the welding spot that is set away from the shadow mask central axis. In other words, this technique fails to achieve corrections on the entire phosphor screen.